1. Field of the Invention
The invention concerns a process for encapsulating active particles within a polymer shell.
2. The Prior Art
There are many particulate active materials which require protection from atmospheric moisture and air or from co-ingredients of compositions with which they are formulated. Examples of such actives include catalysts, bleaches, colorants, enzymes, and similar particles. Some of these actives must not only be protected from the environment but the environment must be protected from the actives. For instance, chlorine releasing bleach particles must be shielded from atmospheric moisture during storage to prevent early decomposition. Conversely, dyes and perfumes formulated with the bleach in a detergent powder must be protected from oxidation by the bleach. Therefore, it may readily be appreciated that coatings encapsulating active particles are important and that the encapsulation be sufficiently coherent to function effectively as a barrier.
Encapsulated solid actives, as suggested above, find use in diverse fields. Although the invention is not so limited, the present process finds special utility in manufacturing encapsulated bleaching related particles for detergent compositions.
There have been a number of encapsulated chlorine bleaches described in the detergent art. U.S. Pat. Nos. 3,908,045, 3,944,497, 3,983,254 and 4,124,734 all to Alterman et al. report the coating of chlorine-releasing particles in a process wherein a non-aqueous solution of a solidifiable fatty acid is sprayed onto a fluidized bed containing the chlorine-releasing particles. U.S. Pat. Nos. 4,078,099, 4,126,717 and 4,136,052 to Mazzola describe a solventless method for coating chlorinating agents. In that process, a rotating drum mixer agitates chlorine-releasing particles while a melted solventless fatty acid is sprayed thereonto. Subsequently, a second coating is applied by spraying a melted solventless mixture of fatty acid, microcrystalline wax and polyoxyethylene-polyoxypropylene copolymer. The patent notes that it is important to maintain an ambient air temperature in the mixer within the softening range of the coating agents to allow the coatings to spread around each particle; more coherent coatings were said to be thereby obtainable.
U.S. Pat. No. 3,112,274 (Morgenthaler et al.) describes a process for making a bleach composition wherein dry polychlorocyanurate particles are suspended in a fluidized bed into which is sprayed an aqueous slurry of an inorganic coating salt. The process emphasizes low drying temperatures ranging from 100.degree. F. to 170.degree. F. outside of which range effective coating is hindered. Another patent, U.S. Pat. No. 3,650,961 (Hudson) practices an addition the reverse of that disclosed in the aforesaid patents. Inorganic coating salts are introduced first into the fluidized bed. Subsequently, an aqueous slurry of chlorocyanurate is sprayed into the bed. Microscopic examination of the resultant particles was said to show the chlorocyanurate to be disposed predominantly at the center while the inorganic salt surrounded the core. British Pat. No. 911,410 (Procter) encapsulates trichloroisocyanuric acid by agitating this material in a revolving drum whereupon is sprayed a solution of lauric acid slurried in carbon tetrachloride.
Fluidized bed coating technology, as evidenced by the aforecited art, requires that the encapsulating wall material be either in a melt or solubilized in some solvent to facilitate delivery into the bed. With regard to the solvent approach, wall materials which are water insoluble require a non-aqueous solvent for delivery. Non-aqueous solvent processes are, however, complicated. They often require handling of flammable solvents, recovery systems, and generally present environmental problems. When very reactive materials are present in the particles to be coated, the handling of solvents becomes especially difficult. Thus, it is desirable in most instances to use water as the carrier to avoid the aforestated complications.
A further problem with certain non-aqueous and aqueous solvent systems is their limited efficiency in delivering high concentrations of encapsulating material. It is desirable to deliver concentrations of 30-50% or more in spray form onto agitated solid actives in a fluid bed. Encapsulating material solubilized at high concentrations in non-aqueous and aqueous mediums frequently results in solutions of extremely high viscosity. Efficient spraying of such high viscosity solutions is very difficult.
Therefore, it is an object of the present invention to provide a process which affords the formation of high-quality, continuous coatings around active particulate materials.
It is another object of the present invention to provide a process wherein the coatings may be applied from an aqueous vehicle which benefits from the non-hazardous nature, handling simplicity and environmental safety associated with water.
A further object of this invention is to provide a high efficiency process characterized by the delivery of encapsulating material in highly concentrated form.